Note: This Synoptic Discussion describes recent weather events and climate anomalies in relation to the phenomena that cause the weather. These phenomena include the jet stream, fronts and low pressure systems that bring precipitation, high pressure systems that bring dry weather, and the mechanisms which control these features — such as El Niño, La Niña, and other oceanic and atmospheric drivers (PNA, NAO, AO, and others). The report may contain more technical language than other components of the State of the Climate series.

NCEI added Alaska climate divisions to its nClimDiv dataset on Friday, March 6, 2015, coincident with the release of the February 2015 monthly monitoring report. For more information on this data, please visit the Alaska Climate Divisions FAQ.

Synoptic Discussion

Monthly upper-level circulation pattern and anomalies.

In the Northern Hemisphere, September marks the beginning of climatological fall which is the time of year when the jet stream becomes more active and spreads cooler air masses from the north across the United States. For September 2013, the subtropical high pressure belt, also called the Bermuda High (or Azores High), dominated the weather during the first half of the month, with warmer-than-average temperatures holding sway across much of the country during the first two weeks (week 1, 2). The Bermuda High retreated during the last half of the month, with a series of upper-level weather systems in the jet stream flow bringing below-normal temperatures to the eastern and western sections of the country. Warmth dominated for the month as a whole, with September 2013 ranking as the sixth warmest September, nationally, in the 1895-2013 record. Seven states, mostly in the Northern Rockies and Northern Plains, ranked in the top ten warmest category for September. There were nearly six times as many record warm daily highs (980) and lows (2294, or a total of about 3300) than record cold daily highs (373) and lows (187, or a total of about 560).

Subtropical highs, and cold fronts and low pressure systems moving in the storm track flow, are influenced by the broadscale atmospheric circulation. The following describes several such large-scale atmospheric circulation drivers and their potential influence this month:

Description:
Oceanic and atmospheric conditions in the tropical Pacific Ocean can influence weather across the globe. ENSO is characterized by two extreme modes: El Niño (warmer-than-normal sea surface temperature [SST] anomalies in the tropical Pacific) and La Niña (cooler-than-normal SST anomalies), with the absence of either of these modes termed "ENSO-neutral" conditions.

Teleconnections (influence on weather):
To the extent teleconnections are known, while in a neutral state, ENSO normally is not a player in the month's weather. Historical data can be analyzed to show typical temperature and precipitation patterns associated with El Niño and La Niña ENSO episodes. Teleconnections are not available for ENSO-neutral conditions.

Teleconnections (influence on weather):
The MJO's temperature and precipitation teleconnections to U.S. weather depend on time of year and MJO phase. To the extent teleconnections are known, the July-September teleconnections for precipitation are shown here and for temperature are shown here.

Observed:
The MJO is transitory and can change phases (modes) within a month, so it is more closely related to weekly weather patterns than monthly. The September 2013 monthly precipitation pattern shows little correlation to the precipitation teleconnection pattern for MJO phases 1 and 2 this time of year, but there is some hint of agreement with dryness in the East for phase 5. The weekly precipitation anomalies (weeks 1, 2, 3, 4) do not show much agreement except for wet conditions in the Northwest during the last week which may correspond to phase 6.
The September 2013 monthly temperature pattern shows some similarity to the teleconnections for MJO phases 1, 2, and 5 (warmer than normal in the West to Central U.S., and normal to cooler than normal in the East). Similar consistency can be seen on a weekly basis (weeks 1, 2, 3, 4) — warm anomaly patterns in the West to Central U.S. during weeks 1 and 2 are consistent with phases 1 and 2, and cool anomalies in the East and Southwest during weeks 3 and 4 are consistent with phases 5 and 6.

Teleconnections (influence on weather):
To the extent teleconnections are known, the temperature teleconnection map for this time of year (October on the maps) shows a transition to above-normal temperatures along the West Coast to Northern Plains and below-normal temperatures in the Southeast with a positive PNA, with near-normal temperatures in between.
The precipitation teleconnection map for this time of year (October on the maps) shows a hint of below-normal conditions in the Great Lakes to Ohio Valley.
The upper-level circulation anomaly teleconnection shows the transition to above-normal heights over the western U.S. and below-normal heights over the southeastern U.S. with a positive PNA.

Observed:
The observed warm anomalies in the central U.S. have little relationship to a positive PNA, or to any phase PNA, this time of year, but drier-than-normal conditions in the Great Lakes were observed. The observed upper-level (500 mb) chart for September 2013 shows above-normal heights over south-central Canada to the north-central U.S. and below-normal heights over the northeast Pacific to Alaska and over the Atlantic just off the eastern U.S. coast. This pattern is similar to the teleconnection for a positive PNA, except it is shifted several degrees longitude to the east.

Observed:
The September 2013 monthly temperature and precipitation anomaly patterns are generally opposite that expected with a negative AO, and the September 2013 upper-level circulation anomaly pattern does not match the teleconnection pattern for a negative AO.

Observed:
The September 2013 precipitation anomaly patterns match those expected for a positive NAO for parts of the Ohio Valley, while the September upper-level circulation anomaly pattern shows little correspondence to that expected for any phase of the NAO.

Description:
The EP-NP teleconnection pattern relates SST and upper-level circulation patterns over the eastern and northern Pacific to temperature, precipitation, and circulation anomalies downstream over North America. Its influence during the winter is not as strong as during the other three seasons.

Status:
The SST pattern over the North Pacific during September 2013 was dominated by a large, but weakening, pool of warmer-than-normal SSTs. The monthly EP-NP index was positive for much of 2013, but turned negative during August and continued negative during September, pulling down the 3-month running mean into negative territory.

Observed:
The September monthly temperature anomalies for Alaska and the contiguous U.S. are consistent with a negative EP-NP, except along the East Coast. The September precipitation anomalies for the Pacific Northwest are consistent with a negative EP-NP, but those for Alaska are not. The September 2013 upper-level circulation anomaly pattern seems shifted slightly westward from that expected with a negative EP-NP, but is generally consistent with it.

Description:
The PT teleconnection pattern relates upper-level circulation over the central subtropical North Pacific to circulation features over the eastern United States. It is most influential during August and September.

Observed:
The September 2013 temperature anomaly pattern matches that for a positive PT across parts of the country, primarily the Mid-Atlantic Coast to Northeast and the Northern Plains to Pacific Northwest. The precipitation anomaly pattern matches that expected for a positive PT for the Midwest but not for Alaska. The September 2013 upper-level circulation anomaly pattern is similar in shape to that expected for a positive PT, but seems shifted a few degrees longitude to the east.

Map of three-month temperature anomalies.

Map of three-month precipitation anomalies.

Upper-level circulation pattern and anomalies averaged for the last three months.

Examination of these circulation indices and their teleconnection patterns, and comparison to observed September 2013 temperature, precipitation, and circulation patterns, suggest that the MJO, PNA, NAO, EP-NP, and PT drivers each exerted some influence on September weather. ENSO was neutral, and thus not a player, and the AO did not seem to be influential. Those drivers associated with the Pacific (MJO, EP-NP, and PT) appeared to exert a controlling influence on the temperature patterns. The Pacific drivers (PNA, EP-NP, and PT) also seemed to have a strong influence on the upper-level circulation. Precipitation is not as strongly correlated to any of the atmospheric drivers during the early fall as it is during the winter, with random convective processes playing a greater role in precipitation during September, but the PNA, NAO, and PT seemed to have a common influence on the Great Lakes to Midwest dryness, while the EP-NP was the likely driver influencing the Pacific Northwest wetness.
This month illustrates how competing atmospheric drivers may work in phase to create a complex weather pattern.

Citing This Report

NOAA National Centers for Environmental Information, State of the Climate: Synoptic Discussion for September 2013, published online October 2013, retrieved on December 9, 2016 from http://www.ncdc.noaa.gov/sotc/synoptic/201309.